Fuel cells for combining hydrogen and oxygen to produce electricity are well known. One class of fuel cell includes a fuel cell element comprising a structural anode, a solid-oxide electrolyte layer, and a cathode layer deposited on the electrolyte layer opposite the anode. In the fuel cell reaction, oxygen anions migrate from the cathode through the electrolyte layer to the anode to combine with hydrogen atoms to produce electricity and water. Such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In a currently-preferred manufacturing sequence for SOFCs, the solid-oxide electrolyte is coated to the anode to yield a so-called “green” bi-layer element, containing typically a dense yttria-stabilized zirconia electrolyte and a porous yttria-stabilized zirconia and nickel cermet anode. The green bi-layer element is fired to burn out various binders and to sinter the ceramic. It is important that the bi-layer element, oversized as formed, be sized to fit into the confines of a fuel cell stack, requiring a high level of dimensional control. It is desired to trim the fuel cell element to particular length and width dimensions at this stage wherein the element comprises only the structural anode and the solid-oxide electrolyte layer.
Various methods have been tried to obtain such dimensional control in both the green form and the post-fired form.
Sizing elements prior to firing has not proved to be successful because of firing shrinkage and variation in firing shrinkage. Fired elements presently have a dimensional standard deviation greater than +/−1% of their pre-fired dimensions.
Sizing elements subsequent to firing has also proved difficult due to the fragile nature of the elements. Any cutting methods contemplated must not induce mechanical stress, especially in a direction perpendicular to the element surface.
Conventional laser cutting of the element has been found to cause undesirable reduction of nickel oxide to nickel in the anode, introducing microcracking and a rough edge finish that leads to mechanical failure mechanisms in the finished fuel cell stack.
Water jet cutting typically includes an abrasive material such as garnet in a high velocity water stream focused at a point. Limitations of this method include garnet contamination of the element and again a rough edge finish as in laser cutting.
What is needed in the art is a means for severing an SOFC bi-layer element that provides a smooth edge, does not induce unacceptable mechanical stresses, and does not produce unacceptable amounts of nickel oxide.
It is a principal object of the present invention to sever a bi-layer SOFC element along a predetermined path, leaving a smooth edge free of microcracks.